1. Field of the Invention
The present invention relates to a water vessel propulsion apparatus for production thrust by means of a water jet or jets.
2. Description of the Related Art
Conventional water jet propulsion apparatuses shown in FIGS. 1 and 2 are well known. In the example shown in FIG. 1, a propulsion water duct 3 extending from a water vessel bottom to a stern thereof is interposed in bent form over the length between an inlet 1 and an outlet 2. The linear space of this duct 3 contains an impeller 26 and a rotative drive shaft 27. The rotative drive shaft 27 is extended through the wall of the duct 3 and coupled with a main engine 15 arranged in the vessel on the extension of the axial line of the shaft 27. In the case of FIG. 2, on the other hand, the propulsion water inlet 1 is suspended below the vessel bottom in the neighborhood of a hydrofoil 28 beneath the water and is open in the forward direction in order to introduce water when the vessel is cruising with the hull 4 levitated above the water surface by means of the hydrofoil 28 at high vessel speed. As described above, the conventional water jet propulsion apparatuses are such that the power of the main engine 15 is transmitted to the impeller 26 through the rotative drive shaft 27, and water is introduced from the duct inlet 1 by pumping due to the rotation of the impeller 26. The water thus pumped is discharged by way of the outlet 2, thereby to propel the vessel by reaction.
The water jet propulsion apparatus constructed as described above has the following problems.
A first problem relates to propulsion efficiency and high-speed performance and is that the bent form of the duct 3 necessarily changes the direction of water flow, thereby proportionally reducing propulsion efficiency. Especially when the vessel is cruising at high speed, a turbulent flow occurs in the bent portion of the duct, thereby increasing the very resistance of the duct 3. In the case of the hydrofoil vessel shown in FIG. 2, the hull 4 of which is floated above the water surface while cruising at high speed, the pump suction height from the water surface to the water jet propulsion apparatus increases. When this vessel is accelerated to levitate the hull 4, the dynamic pressure of the duct inlet 1 is not sufficient until a high cruising speed is reached. This easily causes cavitation by the turbulent flow in the duct 3, thereby reducing the pump efficiency. Also, in order to secure a large thrust for high speed cruising, it is necessary to inject a great amount of water at high rate per unit time for generating a large reaction of the jet. High speed rotation of the impeller 26 for this purpose may generate a cavitation which reduces the pump efficiency. Further, the impeller 26 may be damaged, thereby imposing a limitation on injection speed.
In view of this, such conventional water jet propulsion apparatus uses a pump with a margin of discharge having a large sectional area of the duct 3. The main engine 15 and the propulsion apparatus thus become bulky, and the thrust per unit weight is reduced. It is therefore difficult to increase the propulsion efficiency, thereby imposing a limitation on improvement of high speed performance. Further, the great amount of water injected from the outlet 2 of the propulsion apparatus toward the backward water surface brings with it a great kinetic energy for a reduced propulsion efficiency, resulting in a great energy loss.
A second problem, which relates to the form of the hull, is that since the driving force of the main engine 15 is transmitted to the impeller 26 by the rotative drive shaft 27, the linear arrangement of these components, together with a complicated arrangement of the duct 3, reduces the propulsion efficiency as described above. This also imposes a limitation on relative positions of the duct inlet 1, the outlet 2 and the main engine 15. Therefore, it is difficult to meet requirements of free form of the hull.
A third problem relating to course stability is that the water jet, when injected under water, is rapidly attenuated in flow rate as compared with injection in the air, and the thrust is reduced accordingly. In the conventional water jet propulsion apparatus, the jet is injected on the water surface or at substantially the same height as the water surface, as shown in FIGS. 1 and 2. As a result, the form of the hull shown in FIG. 1 is accompanied by an inferior course stability. Especially when the vessel is cruising at high speed, the hull slides at a high position on the water surface and the water jet is injected with a reduced contact area between the hull 4 and the water, resulting in further reduced course stability.
A fourth problem, which relates to maintenance, is that the bent form of the duct 3, and especially the large total length of the duct 3 in the cases of FIGS. 1 and 2, with movable parts such as the impeller 26 and the rotative drive shaft 27 built into the pump assembly, makes maintenance troublesome. In the case where foreign matter intrudes while the vessel is cruising, the impeller 26 will be damaged. It is therefore necessary to mount a garbage net or the like on the inlet 1 of the duct 3. In high speed craft requiring a high speed pump, in particular, the construction is so complicated by a plurality of impellers and stator vanes that maintenance is quite burdensome.